1999-10-15_GENERAL DOCUMENTS - C1994082

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feet, respectively. Because the drawdowns due to pit inflows will be mostly contained within the leasehold area, and adjacent water users are located beyond the one foot drawdown contours, no significant impact on groundwater use due to pit inflows is predicted. Impacts of Production Well Seneca Coal Company predicted that maximum drawdowns in the Trout Creek aquifer resulting from nine years of pumping in the Yeast Production Well would be 186 feet. After 20 years of recovery, the residual drawdown within one mile of the well is predicted to be less than two feet. The radial distance to the one foot drawdown contour after 20 years of pumping was predicted to be 10 miles. The only known Trout Creek wells within this radius are associated with the Twentymile Coal Mine and are between seven and ten mile from the proposed Yeast Mine well. The drawdown at this distance from the Yeast Mine well should not significantly impact these wells. Streamflow depletions in the Yampa River, Fish Creek, Middle Creek and Trout Creek channels due to changes in the groundwater regime were all predicted to be less than 0.55 acre-feet per year. However, streamflow depletions in Sage Creek and Grassy Creek were predicted to be higher: 4.9 acre-feet per year in Sage Creek and 5.8 acre-feet per year in Grassy Creek. Impact of Replaced•Spoil on Groundwater Seneca Coal Company predicted that the replaced spoil would have a 25% higher hydraulic conductivity than the undisturbed material. Other than that, the primary effect on groundwater would be degradation of quality with respect to total dissolved solids (TDS). Seneca Coal Company predicted that the increase in TDS concentrations in the bedrock aquifers will range from 70% in the Wadge Overburden to over 300% in the Wadge Coal. These predictions are for groundwater in close proximity to the resaturated spoil, within the permit area. As groundwater migrates away from the resaturated spoil, mixing, dilution from unaffected recharge, and geochemical changes will decrease the TDS concentrations. Well yields in these bedrock units, the Wadge Coal and Overburden and the Wolf Creek Coal and Overburden, are insufficient for irrigation purposes. The alluvial aquifers that will be affected by the resaturated spoil include the Annand Draw, Upper Grassy Creek, Lower Grassy Creek, and Sage Creek alluvium. Within the Grassy Creek basin, above the confluence with Scotchman's Gulch, the magnitude of the TDS increase will be high immediately below the resaturated spoil, but will be undetectable a short Yoast Mine 25 October 19, 1999